1. Field of the Invention
The present invention relates to a quartz crystal oscillator provided with a tuning fork type quartz crystal unit, and more particularly to a tuning fork type crystal oscillator that can be used as a clock source, and further, that has an angular velocity detection capability.
2. Description of the Related Art
A tuning fork type crystal oscillator in which a tuning fork type crystal unit and an oscillation circuit that uses this crystal unit are integrated is used as a clock source for generating a reference clock signal in equipment such as a portable telephone. The clock frequency generated by a tuning fork type crystal oscillator is typically 32.768 KHz or 16.384 KHz. In addition to being incorporated in an oscillation circuit and used as a clock source, a tuning fork type crystal unit is also provided with the application of an angular velocity sensor.
In portable telephones, the incorporation of a camera function (i.e., imaging function) as a part of the increased functionality of these devices has become a standard feature. However, portable telephones with this camera function are light in weight and therefore particularly prone to camera shake when the shutter is operated when the camera function is used to take pictures. According to a known technology typically employed in digital cameras or the like, the angular velocity of the camera when taking a picture is measured and the effect of camera shake on the captured image then corrected based on the measured angular velocity. Correction of camera shake can also be implemented in a portable telephone having a camera function by sensing the angular velocity of the portable telephone when taking a picture, and the correction of camera shake by providing an angular velocity sensor that uses a tuning fork type crystal unit can be considered.
FIG. 1 shows the configuration of a conventional portable telephone that is provided with a camera function. This portable telephone is equipped with communication structure 2 for executing communication operations as a portable telephone and camera structure 3 for capturing images. Tuning fork type crystal oscillator 4 is further provided, this tuning fork type crystal oscillator 4 being composed of tuning fork type crystal unit 5 and oscillation circuit 6 that uses this tuning fork type crystal unit 5. Tuning fork type crystal unit 5 is a component having an oscillation frequency of, for example, 32.768 KHz, and oscillation circuit 6 supplies a clock frequency of 32.768 KHz.
Communication structure 2 is controlled by synchronizing to the clock frequency supplied from tuning fork type crystal oscillator 4. Communication structure 2 is connected to antenna 2A and performs transmission to and reception from a base station or the like by way of antenna 2A.
As is well known in the art, three crystallographic axes X, Y, and Z are defined in a quartz crystal. A tuning fork type crystal unit is obtained by cutting a crystal blank in a tuning fork shape from a single crystal of quartz, this crystal blank having a pair of tuning fork arms 7a and 7b and tuning fork base 7c, as shown in FIG. 2A. In this case, a Z-cut wafer in which the principal plane is orthogonal to the Z-axis is used, and the tuning fork type crystal unit is cut from the Z-cut wafer such that the X-axis is the direction of width, the Y-axis is the direction of length, and the Z-axis is the direction of thickness. The horizontal direction shown in the figure is the X-axis direction, and the tuning fork type crystal unit is formed such that the pair of tuning fork arms 7a and 7b each extend in the Y-axis direction from the two ends in the X-direction of tuning fork base 7c. Tuning fork arms 7a and 7b are both shaped as square shafts, and excitation electrodes 8 are formed on each of the four side surfaces of each arm as shown in FIG. 2B.
Regarding excitation electrodes 8 of each tuning fork arm 7a and 7b when this type of tuning fork type crystal unit 5 is excited, the same potential is applied to a pair of opposite excitation electrodes 8 that sandwich the tuning fork arm, and the opposite voltage is applied to the adjacent excitation electrodes 8. At this time, if a negative potential is applied to the two principal planes (of the Z-cut wafer) and a positive potential is applied to the two side planes on one tuning fork arm 7a, a positive potential is applied to the two principal planes and a negative potential is applied to the two side planes on the other tuning fork arm 7b. Crystal unit 5 is provided with a pair of terminals, and lead electrodes (not shown) are provided such that voltage of the same potential and of the opposite potential is applied from the pair of terminals to each excitation electrode 8. The “+” and “−” marks of FIG. 2B show how voltage is to be applied to each of excitation electrodes 8. In FIG. 2B, excitation electrodes 8 marked by “+” are connected in common to one terminal, and excitation electrodes 8 marked by “−” are connected in common to the other terminal.
Next, regarding the operation of this tuning fork type crystal unit, when positive potential and negative potential are applied to excitation electrodes 8 as shown in FIG. 2B, an electric field is produced that is directed from the two side surfaces and toward the two principal surfaces of the Z-cut wafer in one tuning fork arm 7a, and an electric field is produced that is directed from the two principal surfaces and toward the two side surfaces of the Z-cut wafer in the other tuning fork arm 7b, as indicated by the arrows in the figure. In other words, electric fields are produced between the two principal surfaces and two side surfaces are mutually opposed between tuning fork arms 7a and 7b. The Z-cut wafer expands in the Y-axis direction when the electric field component directed in the +X-axis direction is applied, and contracts in the Y-axis direction when a field component toward the −X-axis direction is applied, whereby tuning fork arms 7a and 7b expand in the Y-axis direction on the outer side surface portions of each arm and contract in the Y-axis direction on the inner side surface portions. If the polarity of the potential that is applied to each of excitation electrodes 8 is then reversed, the outer side surface portions of each of tuning fork arms 7a and 7b now contract in the Y-axis direction, and the inner side surface portions expand in the Y-axis direction. By producing this X-Y flexure vibration, the pair of tuning fork arms 7a and 7b generate tuning fork vibration in the horizontal direction in the figure.
Explanation next regards an oscillation circuit that uses this type of tuning fork crystal type unit with reference to FIG. 3. Oscillation circuit 6 is provided with amplifier 8 for oscillation, and a pair of split capacitors 9a and 9b. Amplifier 8 is provided as an inverter amplification element that is provided with feedback resistor 10; and the input end and output end of amplifier 8 are connected to the pair of terminals of tuning fork type crystal unit 5, respectively. Split capacitors 9a and 9b, together with tuning fork type crystal unit 5 as the inductive component, form a resonance circuit, and are each provided between ground and a respective terminal of the pair of terminals of tuning fork type crystal unit 5.
The portion of this oscillation circuit 6 other than crystal unit 5 are provided and integrated in an IC (integrated circuit) chip. After the introduction of the power supply of portable telephone 1, this oscillation circuit constantly generates a clock frequency (for example, 32.768 KHz) to supply oscillation output fout to communication structure 2. The clock frequency is proportional to W/L2, where W is the width and L is the length of the tuning fork arms in tuning fork crystal unit 5.
Explanation next regards camera structure 3. Camera structure 3 is activated by a switch that is separate from the power supply of portable telephone 1, receives angular velocity information from angular velocity sensing mechanism (i.e., angular velocity sensor) 11 for correcting camera shake, and based on this angular velocity information, corrects camera shake when a picture is taken. Angular velocity sensing mechanism 11 is composed of angular velocity sensing element 12, oscillation circuit 6, and detection circuit 13. As shown in FIG. 4, angular velocity sensing element 12 is provided with a tuning fork type crystal blank similar to the previously described tuning fork type crystal unit 5, and excitation electrodes 8 and sensing electrodes 14 that are provided on the side surfaces of the tuning fork arms.
Excitation electrodes 8 are provided on only one tuning fork arm such that the excitation electrodes are disposed on four side surfaces of this tuning fork arm 7a. As in the case shown in FIG. 2B, the same potential is then applied to a pair of opposite excitation electrodes 8 that sandwich tuning fork arm 7a, and the opposite voltage is applied to adjacent excitation electrodes 8. Excitation electrodes 8 on the two side surfaces (of the Z-cut wafer) are connected in common and taken as the first terminal, and excitation electrodes 8 on the two principal surfaces (of the Z-cut wafer) are connected in common and taken as the second terminal, and these first and second terminals are connected to oscillation circuit 6. In this angular velocity sensing element 12, tuning fork arm 7a exhibits flexure vibration when oscillation circuit 6 is put into operation, but tuning fork arm 7b resonates with tuning fork arm 7a and then begins flexure vibration, the entirety of tuning fork arms 7a and 7b thus exhibiting tuning fork vibration. The oscillation frequency in this case is assumed to be approximately 17 KHz, for example.
Two sensing electrodes 14 are provided aligned in the direction of the Y-axis on, of the four side surfaces of tuning fork arm 7b, the two surfaces that are the side surfaces of the Z-cut wafer, i.e., the surface directed in the +X direction and the surface directed in the −X direction. Here, the two sensing electrodes 14 that are provided on each of the pair of side surfaces of tuning fork arm 7b are arranged to confront sensing electrodes 14 on the other side surface with tuning fork arm 7b interposed.
When element 12 rotates with the Y-axis as the center when tuning fork arms 7a and 7b are exhibiting tuning fork oscillation in angular velocity sensing element 12 of this configuration, the Coriolis force acts such that tuning fork arms 7a and 7b exhibit vibration in mutually opposing directions in the up and down directions in the figure with one of tuning fork arms 7a and 7b displaced in the +Y-axis direction and the other displaced in the −Y-axis direction, i.e., such that tuning fork arms 7a and 7b generate X-Z flexure vibration. As a result, electric fields are produced in the X-axis direction at tuning fork arm 7b as shown by the straight arrows in FIG. 4. These electric fields in the X-axis direction are mutually opposing fields at positions close to one principal surface and positions close to the other principal surface of the Z-cut wafer. Electric charge occurs as represented by the “+” and “−” marks shown in the figure in the side surfaces on which sensing electrodes 14 of tuning fork arm 7b are formed, and this charge is detected by sensing electrodes 14. The magnitude of this charge obviously changes in accordance with the magnitude of the angular velocity received by angular velocity sensing element 12. By means of the detection of this charge in detection circuit 13, the angular velocity received by angular velocity sensing element 12 can be determined. Angular velocity signal Sout that corresponds to the detected angular velocity is then supplied as output from detection circuit 13.
An electric field produced by the Coriolis force also occurs in tuning fork arm 7a on which excitation electrodes 8 are provided and an electric charge is produced by this electric field, but the provision of excitation electrodes 8 that cover substantially the entire surface of each side surface of tuning fork arm 7a cancels out the charge produced by the Coriolis force. Accordingly, the influence of the Coriolis force does not reach oscillation circuit 6.
In this angular velocity sensing mechanism 11, a configuration similar to the form described using FIG. 3 is used as oscillation circuit 6. Detection circuit 13 includes a charge amplifier for amplifying the charge that is detected at sensing electrodes 14 and a synchronous detection circuit for carrying out signal processing synchronized to the tuning fork oscillation. Oscillation circuit 6 and detection circuit 13 are integrated as a single unit in IC chip 15, as indicated by the surrounding broken line in FIG. 4.
In a conventional portable telephone such as shown in FIG. 1, tuning fork type crystal oscillator 4 as described hereinabove and angular velocity sensing mechanism 11 are each accommodated in separate surface-mount receptacles and separately packaged and mounted as discrete parts.
Angular velocity sensing mechanism 11 for realizing camera-shake compensation in a camera is provided in a portable telephone of the above-described configuration, but tuning fork type crystal oscillator 4 for use as the reference clock source in the portable telephone and angular velocity sensing mechanism 11 for use in camera-shake compensation both require a tuning fork type crystal unit and an oscillation circuit, and the inclusion of these component interferes with miniaturization of the portable telephone. In addition, the tuning fork type crystal oscillator and angular velocity sensing mechanism, both being separate and discrete parts, raise the problem of an increase in the number of parts.